1. Field of the Invention
This invention relates to an apparatus and method used to design, monitor and evaluate petroleum reservoir fracturing. The invention employs a method to estimate, from available data, the shape of a fracture by way of a numerical simulator which replicates the physical behavior of the hydraulic fracturing process. The size and features of a fracture may be controlled to maximize well productivity following fracturing of the well.
2. Description of the Prior Art
In hydraulic fracturing, thousands of gallons of fluid are forced under high pressure underground to split open the rock in a subterranean formation. Proppant or propping agent is carried into the fracture by the viscosified fluid, and deposited into the fracture. Proppant provides a permeable flow channel for formation fluids such as oil and gas to travel to the wellbore and above the ground surface.
Fracturing involves many variables, including: viscosity of the fracturing fluid, rate of leak-off of fracturing fluid into the reservoir, proppant carrying capacity of the fluid, viscosity of the fluid as a function of temperature, time history of fluid volumes (i.e. the amount of fluid pumped over a given period of time), time history of proppant volumes, fluid physical constants, proppant properties, and the geological properties of various zones in the reservoir.
Currently, fracturing design is accomplished using PC-based programs such as, for example, Schlumberger's FracCADE simulator (FracCADE is a trademark of Schlumberger Technology Corporation). Some of the currently available software has the ability to use what is known in the industry as “pseudo” three dimensional (P3D) hydraulic fracture simulators. Pseudo (P3D) methods are capable of estimating height growth for single fracture geometry's, but cannot accurately represent fracture geometry in more complex treatments that involve multiple geological layers underground (known as “laminated” reservoirs).
Other software has the ability to use what is known in the industry as planar three dimensional (“PL3D”) hydraulic fracture simulators. Methods employing PL3D accurately take into account geologic layers. One such program, known commercially as GOHFER (GOHFER is believed to be a trademark of Stim-Lab and the Marathon Oil Company), provides grid oriented hydraulic fracture replication capabilities. This grid oriented program, and its mode of operation, is seen in FIG. 7. As the front of the fracture moves forward, calculations are made in which each individual grid is either “on” or “off” depending upon whether or not more than half of the individual grid is “covered” by the advancing fracture as it moves outward from the wellbore. If more than one-half of the grid element is covered, then the element is estimated to be fully active. The disadvantage of this system of estimating fracture growth is that it produces too much numerical noise at the fracture tip, and hence in the output data.
Other PL3D methods of simulating fractures include the TerraFrac three dimensional fracturing simulator (TerraFrac is a trademark of the TerraTek Company). This simulator operates as seen in FIG. 8, using estimates that are based upon a method of a moving mesh. This method shows less noise than the GOHFER method, because it uses triangle shaped elements which form a tighter fit with the advancing fracture front. However, it recalculates the entire mesh element set again and again, using large amounts of computing power.
What is needed in the industry is a software implemented method that is capable of accurately and efficiently estimating a fracturing event—before the event, during the event, or after the event. A method and process is needed that is capable of properly accounting for all kinds of layer contrasts, including elastic properties, layer toughness, leakoff parameters, and confining stress; A method is needed which can estimate pinch point scenarios, estimate runaway height growth, and join or separate fractures in the same plane. A process is needed that does not require periodic “re-meshing”, as typically is required with many prior art moving mesh techniques. A technique in which only the fracture front is tracked is highly desirable. Further, a system which is not limited to only an “on/off” binary system for elements would be beneficial. A system that facilitates rapid updates of the solution at each step is needed.